In a typical Scotch yoke mechanism, a yoke frame presents a pair of parallel inner tracks between which a roller or slide block operates in reciprocating motion. In its simplest form the yoke frame moves linearly in simple harmonic motion in a direction transverse of the tracks, and the reciprocating element in the yoke frame is pivotally connected to a rotating crank. Commonly, the yoke frame is rigidly connected to a piston rod and the reciprocating crank is on a crankshaft. When the reciprocating element is a roller, during each stroke the roller engages one of the yoke frame tracks when rolling in one direction and engages the other track when rolling in the opposite direction. Even though the tolerance between the roller diameter and the distance between the tracks is small, the roller impacts the track to which it is shifting. Typical Scotch yoke mechanisms using a roller as the reciprocating element in the yoke frame, and serving as a linkage between a crankshaft and a piston, are shown in U.S. Pat. Nos. 2,132,802 (Pierce) and 2,482,967 (Cook).
It has been discovered that in such mechanisms the roller shifts tracks before the end of its stroke and hence is forced to instantaneously change direction of rotation while finishing the stroke in engagement with the other track. Then, at the finish of the stroke, the roller is again forced to reverse its rotational direction. As a consequence, a scuffing action occurs between the roller and tracks during each stroke.
When a slide block is used as the reciprocating element in the yoke frame, because of the tolerance between the block width and the track spacing, the slide block cocks slightly until its trailing edge and diagonally opposite forward edge engage the two tracks; and when the slide block reverses direction, it cocks oppositely. This action is sometimes referred to in the art as "wedging" and results in scuffing and wear problems. Typical Scotch yoke mechanisms using a slide block as the reciprocating element in the yoke frame and functioning between a crankshaft and a piston are shown in U.S Pat. Nos. 963,880 (Eason); 1,316,192 (Sawyer); 2,127,729 (Grant) and 2,148,820 (Tucker).
In the Tucker patent, the crankshaft has a central crankpin with a slide block operating in a central yoke frame coupled to the rod of an inner piston and has a pair of outer crankpins with slide blocks operating in outer yoke frames coupled by sets of rods to an intermediate piston opposing the inner piston and an outer piston opposing an end of the crankcase. The Tucker patent also shows a double-cylinder engine arrangement in which the outer yoke frames each have a second set of rods extending oppositely from the first set. It has been discovered that with such an arrangement of outer yoke frames, respective tracks in the outer yoke frames tend to move out of parallel relationship to one another and to distort under load.
Another problem with Scotch yoke mechanisms is lubrication of the bearing for the roller or slide block providing connection with the crankpin on which the roller or slide block is mounted. This is particularly true in high speed and high thermal operating conditions and has been a limiting factor in power transfer design, particularly when external heat engines are given consideration. It has been discovered that under such thermal operating conditions that solid lubricated antifriction roller bearings will allow significantly longer operation of the scotch yoke mechanism particularly at higher temperatures.
Yet another problem is the tendency of the reciprocating scotch yoke to rotate about the reciprocation axis and thereby cause additional scuff and wear as the side edges of the scotch yoke collides with the webs of the crankshaft during operation.